Several available devices convert mechanical energy in the local environment into electrical energy, including the Seiko “Kinetic” watch and mechanical wind-up radios. An article, “Energy Scavenging with Shoe-Mounted Piezoelectrics,” by N. S. Shenck and J. A Paradiso reports on systems that capture energy from the user's environment to provide electricity to wearable microelectronic devices without batteries. The unobtrusive devices scavenge electricity from the forces exerted on a shoe during walking. The devices include a flexible piezoelectric foil stave to harness sole-bending energy and a reinforced piezoelectric dimorph to capture heel-strike energy. They also report on prototype development of radio frequency identification (RFID) tags which are self powered by a pair of sneakers.6 A recent report by Meniger et al., entitled “Vibration-to-Energy Conversion”, discloses a microelectromechanical system (MEMs) device for the conversion of ambient mechanical vibration into electrical energy through the use of a variable capacitor However, these MEMs systems only demonstrated 8 microwatts of power. Transmission of RF data over distances of 20 feet or more requires milliwatt power levels.
Low power sensors have been developed, as described on commonly assigned U.S. patent application Ser. No. 09/731,066, to Arms, that includes a sensing unit for attaching to a structure or live subject for sensing a parameter of the structure or live subject. The sensing unit includes a sensor, a data storage device, and a transmitting device. The data storage device is for storing data from the sensor. Power is provided by a power supply such as a rechargeable battery or fuel cell. The rechargeable battery can be recharged by inductive coupling from an external control unit.
Over the past years, sensors, signal conditioners, processors, and digital wireless radio frequency (RF) links have become smaller, consumed less power, and included higher levels of integration. The Ser. No. 09/731,066 application, for example, provides sensing, acquisition, storage, and reporting functions. Wireless networks coupled with intelligent sensors and distributed computing have enabled a new paradigm of machine monitoring.
A paper, “Wireless Inductive Robotic Inspection of Structures,” by Esser, et al, proceedings of the IASTED International Conference, Robotics and Applications 2000, August 14-16, 2000, Honolulu, Hi., describes an autonomous robotic structural inspection system capable of remote powering and data collection from a network of embedded sensing nodes and providing remote data access via the internet. The system uses microminiature, multichannel, wireless programmable addressable sensing modules to sample data from a variety of sensors. The nodes are inductively powered, eliminating the need for batteries or interconnectLng lead wires.
Wireless sensors have the advantage of eliminating the cost of installing wiring. They also improve reliability by eliminating connector problems. However, wireless sensors still require system power in order to operate. If power outages occur, critical data collected by the sensors may be lost. In some cases, sensors may be hardwired to a vehicle's power system. In other cases however, the need to hard wire to a power system defeats the advantages of wireless sensors, and this may be unacceptable for many applications. Most prior wireless structural monitoring systems have therefore relied on continuous power supplied by batteries. For example, in 1972, Weiss developed a battery powered inductive strain measurement system, which measured and counted strain levels for aircraft fatigue. Traditional batteries, however, become depleted and must be periodically replaced or recharged, adding an additional maintenance task that must be performed. This is particularly a problem for monitors used for a condition based maintenance program since it adds additional maintenance for the condition based monitoring system itself.
None of the systems for sensing changes in the environment have collected available mechanical energy to provide the electricity for running the sensors, storing data from the sensor, or communicating the data externally. Thus, a better system for powering sensors and storage devices, and for transmitting data gathered by sensors is needed, and this solution is provided by the following invention.